Add the four settings the README documented but pixa lacked (closes #61)

README.md documented access_control_allow_origin,
upstream_fetch_timeout, upstream_max_response_size and
downstream_timeout, but pixa did not know them, so a config following
the README aborted startup. Each is now a setting with its PIXA_
variable, defaulting to the value that was fixed in the code: *, 30s,
50 MiB and 60s. Durations are Go duration strings and must be positive;
the size is whole bytes, at most 1 GiB. The origin is * or one http or
https origin written exactly as a browser sends it; anything else
aborts startup. downstream_timeout sets both the server's write timeout
and the per-request timeout. The owner approved the edits to existing
tests.

Model: opus-5-5

internal/config/env_internal_test.go

@@ -8,6 +8,7 @@ import (
 	"slices"
 	"strings"
 	"testing"
+	"time"
 
 	"sneak.berlin/go/pixa/internal/globals"
 	"sneak.berlin/go/pixa/internal/logger"
@@ -69,6 +70,10 @@ func TestEnvironmentSetsEveryKey(t *testing.T) {
 	t.Setenv("PIXA_CACHE_MAX_BYTES", "1024")
 	t.Setenv("PIXA_BLOCKED_NETWORKS", "203.0.113.0/24")
 	t.Setenv("PIXA_TRUSTED_PROXIES", "192.0.2.0/24")
+	t.Setenv("PIXA_ACCESS_CONTROL_ALLOW_ORIGIN", "https://app.example.com")
+	t.Setenv("PIXA_UPSTREAM_FETCH_TIMEOUT", "10s")
+	t.Setenv("PIXA_UPSTREAM_MAX_RESPONSE_SIZE", "1048576")
+	t.Setenv("PIXA_DOWNSTREAM_TIMEOUT", "2m")
 
 	c, err := newFromSmartConfig(nil)
 	if err != nil {
@@ -92,6 +97,10 @@ func TestEnvironmentSetsEveryKey(t *testing.T) {
 		cacheMaxBytesExplicit:      true,
 		BlockedNetworks:            []netip.Prefix{netip.MustParsePrefix("203.0.113.0/24")},
 		TrustedProxies:             []netip.Prefix{netip.MustParsePrefix("192.0.2.0/24")},
+		AccessControlAllowOrigin:   "https://app.example.com",
+		UpstreamFetchTimeout:       10 * time.Second,
+		UpstreamMaxResponseSize:    1048576,
+		DownstreamTimeout:          2 * time.Minute,
 	}
 
 	if !reflect.DeepEqual(*c, want) {
@@ -280,6 +289,30 @@ func TestInvalidDebugFromEnvironmentAbortsStartup(t *testing.T) {
 	wantStartupError(t, err, "PIXA_DEBUG", "maybe")
 }
 
+// TestInvalidOriginTimeoutOrSizeFromEnvironmentAbortsStartup checks that
+// an invalid CORS origin, timeout or response size limit in its variable
+// aborts startup naming the variable and the value.
+func TestInvalidOriginTimeoutOrSizeFromEnvironmentAbortsStartup(t *testing.T) {
+	cases := []struct {
+		variable string
+		value    string
+	}{
+		{"PIXA_ACCESS_CONTROL_ALLOW_ORIGIN", "example.com"},
+		{"PIXA_UPSTREAM_FETCH_TIMEOUT", "soon"},
+		{"PIXA_UPSTREAM_MAX_RESPONSE_SIZE", "50MB"},
+		{"PIXA_DOWNSTREAM_TIMEOUT", "0s"},
+	}
+
+	for _, tc := range cases {
+		t.Run(tc.variable, func(t *testing.T) {
+			t.Setenv(tc.variable, tc.value)
+
+			_, err := configFromYAML(t, signingKeyLine)
+			wantStartupError(t, err, tc.variable, tc.value)
+		})
+	}
+}
+
 // TestConfigFileAloneBehavesAsBefore checks that with no variables set
 // (TestMain unsets them) the config file's values are used and omitted
 // keys take their defaults.